Plate-fin coil air-side surfaces are formed in progressive dies. There are several variants of these dies which include draw forming, drawless forming, fin-per stroke, and high collar dies. For each method, a primary consideration is the formation of the tube contact cylinder of the fin collar, which is used as the contact area between the fin collar and the heat exchanger tube. From both thermal performance and corrosion durability perspectives, a greater contact area is advantageous. Also, for many applications a high fin density is desirable. Therefore, it is preferable to have a large number of fin collars with a relatively small size contact leg, but with a large percentage of the contact leg in contact with the heat exchanger tube. Also, the manufacturing process should be flexible in making fin sizes for a wide range of fins per inch and capable of producing a good and repeatable collar geometry. Current methods fail to adequately achieve these goals. As represented in FIGS. 4 and 4a, most fin collars formed in accordance with prior art methods have tube-contact legs which only contact the tube surface over a very short distance, essentially at the apex of the contact leg's radius.
For a coil made with bare finstock, a relatively small contact area between fin and tube will provide thermal transport with minimal thermal resistance. However, if the finstock has an organic film or other coating with a significant thermal resistance, a larger contact area provides substantially improved performance.
With current practices, while the length of the contact leg is somewhat adjustable or flexible, based on the ability to perform multiple drawing stages, the resulting contact leg is frequently not formed sufficiently straight. The limitations of various current fin forming methods can be seen by referring to FIG. 5. The fin collar formed from this method includes contact legs that are curved and do not effectively cover the surface of the heat exchanger tube, as shown in FIG. 4a, thereby inefficiently contacting the tube surface and accordingly, failing to achieve the best heat exchange relationship therewith.
More specifically, in the draw forming method of FIG. 5a, a sheet or strip of fin stock material is formed with a button therein. The height or depth of the button may be increased or decreased to adjust the fin density and the length of the fin collar contact leg. Accordingly, a number of drawing stages are used to shape the contact leg of the fin collar. The button is then pierced and the fin collar is shaped, straightened and flared for forming the desired contact leg. Corrosion durability of an aluminum fin/copper tube heat exchanger is inversely proportional to the exposed area of the copper tube in the fin pack of the coil. This is because the primary corrosion mechanism for these heat exchangers is galvanic corrosion. Reducing the cathodic copper area proportionally decreases the corrosion current. In addition, improving the straightness of the collar contact area decreases access of electrolyte to the copper/aluminum contact area of the galvanic couple. More complete coverage of the tubes by the aluminum collar improves corrosion durability. The amount of electrolyte that can be stored in the collar crevice is also a function of the collar design. The reduction in the electrolyte content proportionately reduces the galvanic current.
The drawless forming method of FIG. 5b begins with a piercing and burling step and thereby lacks the multiple drawing stages of the draw forming method and, accordingly, lacks the flexibility of adjusting the contact leg length. In the first step, fin stock is pierced and buried to form a pre-contact leg. The pre-contact leg is ironed for straightening and limited lengthening and finally, the tip of the leg is flared or curled. Accordingly, this method lacks the flexibility of adjusting the contact leg length. Similarly, the single shot method shown in FIG. 5c also lacks flexibility, starting with a piercing step, then a burling step to bend and form the pre-contact legs, and finally a flaring step for flaring or curling the ends of the contact legs. The high fin method of FIG. 5d has substantially the same steps as the draw forming method with additional ironing steps between the piercing and burling and flaring steps so as to somewhat improve the straightness of the contact leg. However, the high fin method suffers from the same defects or shortcomings as the draw forming method, described above.
There exists a need, therefore, for an improved fin collar forming method whereby the fin collar is formed with a substantially straight contact leg and greater contact area and whereby the method has the flexibility to provide for any desired length of the contact leg while maintaining its straightness as well as good physical and material characteristics.